This invention relates to a novel powderless etching bath composition for etching plates of magnesium or alloys thereof, an additive for use with nitric acid and water to form an etching bath formulation, and a method for etching plates of magnesium or magnesium alloy. (Hereinafter the term "magnesium" includes alloys thereof containing at least about 70 weight percent magnesium.)
In the preparation of metal printing plates, a plate of an acid soluble metal such as magnesium is coated in the plate making plant with a light sensitive coating or "enamel" which is applied to the metal surface which is usually flat but may be curved. Alternatively, photoresist coated ("presensitized") flat plates are now available commercially which enable one practicing the art to avoid the time consuming coating step immediately prior to exposing the plate. The coated surface is exposed to light through a negative having an image thereon so as to cause or initiate cross-linking in the polymeric coating material to produce a corresponding image in the coating. The exposed coated surface is developed, and a pattern of an acid resistant coating is thereby formed on the plate in the form of the image produced by the exposure and is known as the "photoresist" image, or simply "resist" image. Depending on the type of coating employed, an additional heating step may be required to further harden and affix the acid resistant coating to the metal surface. Subsequently, the image bearing surface of the plate is etched by contacting it with an acid, thereby dissolving metal from those portions of the plate which are not protected by the photoresist. In the absence of any means of sidewall protection, the acid also tends to etch laterally and undercut the resist as the etching proceeds, thus distorting or destroying the image.
Originally, this undercutting was controlled by the time consuming and difficult method known as "powdering" of the plate, whereby the sides of the relief were manually covered with an acid resistant powder several times during the etching process.
Approximately 20 years ago, powderless etching baths usable in etching machines were introduced as taught in U.S. Pat. Nos. 2,640,763-'767 which were so formulated as to eliminate the need for powdering. Though at the time they represented a revolutionary advance over the prior art, these early formulations were deficient in many respects.
The art of powderless etching has continued to be refined and advanced in recent years, and is now a well-known, widely practiced technique. Subsequent formulations, such as that taught in U.S. Pat. No. 2,979,387, for example, employed large quantities of a water immiscible organic liquid which is substantially unreactive with the aqueous acid bath. However, the modern formulations employing such organic liquid are undesirable in that they pose a substantial waste disposal problem. Since the water immiscible organic liquid, which is generally a petroleum solvent, is a serious pollutant, it is ecologically unsound to dispose of the used bath by merely draining it in a conventional drain. Very recently, a series of five patents have issued (U.S. Pat. Nos. 3,725,158; 3,725,159; 3,728,180; 3,730,899; and 3,736.197), which teach new formulations whereby the water immiscible organic liquid can be eliminated or at least reduced. The etching bath of each of these recent patents employs a sulfonated carboxylic acid.
It is an important object of this invention to provided a novel bath for etching at high speeds without sacrificing the quality of the etch produced, shaped articles of magnesium or magnesium alloys such as photoengraving plates.
It is another object of this invention to provide an improved, high speed etching bath having a markedly improved filming capacity for better side wall protection.
Other objectives of this invention include providing a novel high speed etching bath capable of producing desired depths of etch in all kinds of image areas of combination plates, including solid areas, half tones, and large open areas, providing a high speed etching bath capable of forming a stable protective film to prevent or retard lateral etching and chipping of relief side walls; and providing a high speed etching bath which eliminates or minimizes the so-called "pimpling" in large open areas.
Two additional objectives of this invention are to provide a novel additive for used in connection with water and nitric acid in making up etching baths and to provide a novel method for high speed etching.
Finally, it is a primary objective to accomplish all of the above as well as arriving at a stable composition for enhanced bath life without employing an unreactive water immiscible organic liquid.
Other and additional objects will become apparent from a consideration of this entire specification, including the claims.